This invention relates to a method of producing coloured diamond and more particularly coloured single crystal chemical vapour deposition (hereinafter referred to as CVD) diamond that is suitable, for example, for ornamental purposes.
Intrinsic diamond has an indirect band gap of 5.5 eV and is transparent in the visible part of the spectrum. Introducing defects or colour centres, as they will be called hereinafter, which have associated energy levels within the band gap gives the diamond a characteristic colour that is dependent on the type and concentration of the colour centres. This colour can result from either absorption or photoluminescence or some combination of these two. One example of a common colour centre present in synthetic diamond is nitrogen which, when on a substitutional lattice site in the neutral charge state, has an associated energy level ˜1.7 eV below the conduction band—the resulting absorption gives the diamond a characteristic yellow/brown colour.
It is well known that post-growth treatment of diamond, such as irradiation with sufficiently energetic particles or radiation (electron, neutron, gamma etc) to produce lattice defects (interstitials and vacancies) and suitable subsequent annealing, can result in the formation of colour centres such as the nitrogen vacancy [N-V] colour centre which can give the diamond a desirable colour (see for example EP 0 615 954 A1, EP 0 326 856 A1 and the references cited therein). Further characteristics and artificial production of colour centres are discussed in detail by John Walker in the Reports on Progress in Physics, Vol. 42 1979. The artificial production method of creating colour centres outlined in these reports comprises the steps of forming lattice defects in crystals by electron beam irradiation and, if necessary, performing annealing to cause the lattice defects to combine with nitrogen atoms contained in the crystals. However, there are limitations to the colours and uniformity that can be produced as a consequence of competitive defect formation and because of the strong growth sector dependence associated with the concentration of defects such as nitrogen in diamond.
The colour of a diamond coloured by utilising a post growth colour centre formation method is the colour of the rough diamond prior to post growth treatment combined with the effect on colour of the one or more colour centres modified or produced during post growth treatment. In order to obtain the ornamental value desired, and thus achieve a combination of high transparency and desirable colour, it has been usual practice to use diamonds that were initially either colourless or light yellow. This method is therefore not readily applicable to brown single crystal CVD diamond.
EP 671482, U.S. Pat. No. 5,672,395 and U.S. Pat. No. 5,451,430 describe methods of making polycrystalline CVD diamond more transparent using an HPHT treatment that densities the diamond.
It is also known that the colour of brown natural diamond can be altered by annealing at high pressures and temperatures. For example, natural type IIa diamond can be made colourless by annealing at very high temperatures under stabilising pressure or it may be turned pink by annealing at rather lower temperatures, again under stabilising pressure. Brown colour in natural diamond is believed to be associated with plastic deformation but the exact cause of the brown colour and how it is modified by annealing is, however, still unknown.
There are three visual attributes to colour: hue, lightness and saturation. Hue is the attribute of colour that allows it to be classified as red, green, blue, yellow, black or white, or a hue that is intermediate between adjacent pairs or triplets of these basic hues.
White, grey and black objects are differentiated on a lightness scale of light to dark. Lightness is the attribute of colour that is defined by the degree of similarity with a neutral achromatic scale starting with white and progressing through darker levels of grey and ending with black.
Saturation is the attribute of colour that is defined by the degree of difference from an achromatic colour of the same lightness. It is also a descriptive term corresponding to the strength of a colour. The diamond trade uses adjectives such as intense, strong and vivid to denote different degrees of saturation assessed visually. In the CIE L*a*b* colour system, saturation is the degree of departure from the neutral colour axis (defined by saturation=[(a*)2+(b*)2]1/2, see hereinafter). Lightness is a visual quality perceived separately from saturation.
Methods of depositing material such as diamond on a substrate by CVD are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F,Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen, and for boron. Nitrogen can be introduced in the synthesis plasma in many forms; typically these are N2, NH3, air and N2H4. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy wherein x and y can each be 1 to 10 or halocarbons CxHyHalz wherein x and z can each be 1 to 10 and y can be 0 to 10 and optionally one or more of the following: COx, wherein x can be 0.5 to 2, O2, H2, N2, NH3, B2H6 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled; for example hydrogen may be present as deuterium or tritium, and carbon may be present as 12C or 13C. Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF (radio frequency) energy, a flame, a hot filament or jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
CVD diamond may be produced on a variety of substrates. Depending on the nature of the substrate and details of the process chemistry, polycrystalline or single crystal CVD diamond may be produced.